Various blade configurations have been proposed in such a coupling for the purpose of increasing its coupling factor (i.e. its braking effect in the case of a retarder) and/or for intensifying the flow of hydraulic liquid which conventionally can be used for cooling purposes; see for example U.S. Pat. No. 2,126,751. In some instances, the blade is provided with a facet at its free edge in order to sharpen the latter, e.g. as shown in German Pat. No. 1,625,836. Other wheel structures, exemplified by German published specification No. 2,239,562, have blades whose leading and trailing flanks gradually converge toward their free edges.
As long as the blades are substantially centered in radial planes, a wedge-shaped blade profile facilitates the casting of the entire blade assembly in a unitary mold. This, however, is difficult or impossible if the blades are tilted forwardly for increased efficiency, i.e. if the free edge of the blade is offset from its root in the direction of rotation relative to the other wheel. When a split mold is used, manufacturing costs are increased and the resulting burrs or ridges on the blade must be removed by separate machining so as not to interfere with the fluid flow. Naturally, any sharpening of the blade edges, as discussed above, also calls for a special machining operation.